The present invention generally relates to electronic security systems, and in particular, to an improved electronic article surveillance system.
A variety of electronic article surveillance systems have been proposed and implemented to restrict the unauthorized removal of articles from a particular premises. One common form of this is the electronic article surveillance system which has come to be placed near the exits of retail establishments, libraries and the like. However, electronic article surveillance systems are also used for purposes of process and inventory controls, to track articles as they pass through a particular system, among other applications.
Irrespective of the application involved, such electronic article surveillance systems generally operate upon a common principle. Articles to be monitored are provided with tags (of various different types) which contain a circuit (a resonant circuit) for reacting with an applied radio-frequency field. A transmitter and a transmitting antenna are provided to develop this applied field, and a receiver and a receiving antenna are provided to detect disturbances in the applied field. If the resonant circuit of a tag is passed between the transmitting and receiving antennas (which are generally placed near the point of exit from a given premises), the applied field is affected in such fashion that a detectable event is produced within the receiver. This is then used to produce an appropriate alarm. Systems of this general type are available from manufacturers such as Checkpoint Systems, Inc., of Thorofare, N.J., among others.
Although such systems have proven effective in both security as well as inventory and process management, it has been found that certain improvements to such systems would be desirable. Perhaps foremost is the ever-present desire to reduce to the extent possible any errors (e.g., false alarms) which are produced by such systems, particularly in terms of their discrimination between the presence of a tag (signifying the presence of a protected article) and other interference which may be present in the vicinity of the electronic article surveillance system. Any steps which can be taken to improve the accuracy of the system will tend to reduce such undesirable results.
More recently, it has become of interest to provide an electronic article surveillance system with sufficient resolution to actually distinguish between different types of tags, resulting from differences in the resonant circuits which they contain. It has long been recognized that different types of tags have different "signatures" (responses) corresponding to the configuration of the resonant circuits which they contain. For example, the resonant circuit of a so-called "hard" tag will generally tend to produce a signal which is somewhat stronger than other types of tags, such as hang-tags and labels, resulting from differences in the size and configuration of the components which comprise these particular labeling devices. As a result, it becomes conceptually possible to differentiate between these various types of tags and labels by analyzing their signatures, by discriminating between the different signals which are possible. However, to date, available systems did not possess the sensitivity to detect these differences in a reliable fashion.